This application claims the benefit of German application no. 100 49 557.5 filed Oct. 6, 2000.
1. Field of the Invention
The invention relates to a device for converting the intensity distribution of a laser beam wherein the device is particularly suitable for generating a laser beam with an intensity which falls constantly along a transverse axis from one side of the beam to the other.
In the present application constant fall (or constant rise) in intensity is not to be considered as a mathematically strictly monotonous fall (or monotonous rise) but as a change in intensity on a scale which is of technical significance for the use of the laser beam, i.e. intensity fluctuations over smaller regions can exist but these are of no importance to the technical outcome in the use of the laser beam and in no way alter the basic tendency of the change in the intensity distribution.
2. Description of the Related Art
In numerous applications of lasers it is desirable to use a laser beam which has an inhomogeneous beam distribution. For example lasers can be used for the recrystallisation of amorphous Sixe2x80x94layers by passing a laser beam over substrates containing these layers. For a homogeneous recrystallisation it has proved adcantageous for the leading edge of the passing laser beam to possess an increased intensity compared to the following part of the laser beam.
In the prior art it is known to control the intensity distribution of a laser beam by extracting parts thereof. For example DE 199 15 000 has disclosed a device wherein sub-beams of the laser beam are mutually superimposed such that a homogenisation of the intensity distribution of the laser beam is fundamentally promoted, but in the case of those sub-beams which are not completely mutually superimposed a diaphragm in the beam path of the laser beam extracts different components. Laser beam power is always lost upon the extraction of parts of the laser beam.
The object of the present invention is to provide a device with which it is possible to generate an intensity profile of the laser beam which, transversely to the laser beam direction, has a higher intensity on one side of the beam than in other regions of the beam. In particular the laser beam power irradiated into the device is to be utilized in as loss-free a manner as possible.
The object is achieved by a device for converting the intensity distribution of a laser beam with
a homogenizer which mutually superimposes sub-beams of the laser beam such that when a laser beam passes through the homogenizer, a homogenization of the intensity distribution of the laser beam is promoted and
at least one inverter which inverts the intensity distribution at least of one sub-region of the laser beam in the direction transverse to the direction of the laser beam.
Here inversion of intensity along an axis or in a direction is to be understood in that the sequence of the intensity values of the sub-region is exactly reversed from one side of the sub-region to the other. If for example the sub-region has an intensity a at the left-hand margin (here the orientation has been selected at random), an intensity b in the centre, and an intensity c at the right-hand margin, the inverted sub-region has the intensity c at the left-hand margin, the intensity b in the centre, and the intensity a at the right-hand margin.
A homogenizer as used in the device according to the invention is known for example from DE 42 20 705 A1 wherein the intensity distribution of a laser beam is homogenized (spatially adjusted) in that a plurality of lenses are arranged in a row at right angles to the optical axis. These lenses are each shaped such that they mutually superimpose individual sub-beams of the laser beam such that the total focused laser radiation is substantially homogenized.
A further development of such a homogenizer is described in DE 196 32 460 C1 wherein a plurality of illuminating fields, each having a homogeneous intensity distribution, are generated, and wherein a row of lenses comprises a plurality of different groups of acentric lens segments of cylindrical lenses (see also U.S. Pat. No. 5,796,521).
In accordance with an advantageous development of the device according to the invention, the inverter consists of two converging lenses with the focal lengths f5 and f4, which are spaced from one another by the distance f5+f4 in the direction of the beam propagation, i.e. in a telescopic arrangement.
As is known, a converging lens is sufficient to invert an intensity distribution. (A converging lens forms an object on a screen arranged behind its focal point upside down). The second converging lens provides that beams impinging in parallel on the first converging lens also issue from the inverter in parallel in the direction of the beam propagation.
Preferably the inverter is arranged in front of the homogenizer in the direction of the beam propagation (on the beam axis).
Preferably the sub-region of the laser beam which is inverted by the inverter is one of the sub-beams of the laser beam which is (subsequently) superimposed by the homogenizer. The inverter can also invert a group of two or more adjacent sub-beams which are (subsequently) superimposed by the homogenizer. If for example the one sub-beam has an intensity a at the left-hand margin (here again the orientation has been selected at random), an intensity b in the centre and an intensity c at the right-hand margin, and the adjacent sub-beam has the intensity c at the left-hand margin, an intensity d in the centre and an intensity e at the right-hand margin, following the inversion the sub-region has the intensity e at the left-hand margin, then the intensity d, then the intensity c, then the intensity b and at the right-hand margin has the intensity a.
It is also possible to provide two inverters which each invert one sub-region or sub-beam. Then, in the above example of the adjacent sub-beams, following the inversion one does not obtain an intensity curve extending from the intensity e to the intensity a, but rather the first sub-beam is inverted independently of the adjacent sub-beam so that following the inversion of the two sub-beams, the sub-region has the intensity c at the left-hand margin, then the intensity b and then the intensity a, from which the intensity jumps to the intensity e, whereupon further to the right the sub-region has the intensity d and at the right-hand margin again has the intensity c.
The above described invention combines a known homogenizer with an additional inverter.
In accordance with an alternative embodiment of the invention, a known homogenizer can be extended such that, as previously, the sub-beams passing through said homogenizer are mutually superimposed but at least one of the sub-beams is inverted once (or three times, five times etc.) more than the other sub-beams.
Thus a variant of the invention relates to a device for converting the intensity distribution of a laser beam comprising a plurality of optical appliances and a converging lens arranged behind them, which are selected and arranged such that each optical appliance focuses one sub-beam of the laser beam, and that having passed through the device the sub-beams are mutually superimposed, which device is characterised in that at least one of the optical appliances is designed such that along at least one axis, along which the intensity is to be converted, it inverts the associated sub-beam once more than the other optical appliances invert their associated sub-beams.
For example, the optical appliances generally each comprise one pair of lenses and the optical appliances which invert the associated sub-beam once more than the other optical appliances comprise two pairs of lenses. Of these pairs of lenses, one pair can consist of two converging lenses with the focal lengths f5 and f4 which are spaced from one another by the distance f5+f4 in the direction of the beam propagation. The second pair of lenses of this optical appliance then for example has focal lengths which are smaller than in the case of the other optical appliances so that sufficient space is available for the two pairs of lenses.
The forementioned devices according to the two alternative embodiments can be extended by combination with a laser beam source, which generates a laser beam with an intensity distribution which satisfies specific requirements, to form a device for generating a laser beam with an intensity which falls constantly from one side to another along an axis extending transversely to the beam direction.
In accordance with the invention, a device of this kind comprises:
a laser beam source which generates a laser beam in which the intensity in selected sub-beams of the laser beam rises constantly along the axis from one side of the selected sub-beams or sub-regions to the other side thereof,
a least one inverter which inverts the intensity distribution at least of one of the selected sub-beams or sub-regions along the axis and
a homogenizer which mutually superimposes sub-beams of the laser beam such that the laser beam would be homogenized if none of its sub-beams were to pass through the inverter.
It is possible for the homogenizer to invert the sub-beams once again. In this case the roles of the xe2x80x9conexe2x80x9d side and the xe2x80x9cotherxe2x80x9d side of the sub-beam relative to the xe2x80x9conexe2x80x9d side and the xe2x80x9cotherxe2x80x9d side of the generated total beam are transposed.
Advantageously, the laser beam source generates a laser beam whose intensity distribution firstly rises constantly to a maximum from one side (or edge or margin) of the laser beam along the transverse axis and falls constantly from the maximum towards the other side. In this case the inverter(s) is/are advantageously arranged such that an inversion takes place only of sub-regions or sub-beams on one side of the maximum. Typically the laser beam source is an excimer laser beam source which generates a symmetrical intensity distribution which in particular is bell-shaped along one of the axes. (Along the other axis the intensity distribution in excimer lasers is generally more or less homogeneous).
The invention further relates to a method of generating a laser beam with an intensity distribution which falls constantly along an axis from one side to the other, comprising the following steps:
irradiation of a laser beam in which the intensity distribution in selected sub-regions or sub-beams of the laser beam rises constantly along the axis from one side of the selected sub-regions or sub-beams to the other side thereof,
inversion at least of one of the selected sub-regions/sub-beams along the axis,
passage of the laser beam through a homogenizer which mutually superimposes sub-beams of the laser beam such that without the step of the inversion at least of one of the selected sub-regions/sub-beams, a homogenization of the intensity distribution would be promoted.
Advantageously, the step of the inversion of the at least one of the selected sub-beams takes place (temporally and spatially) before the passage of the laser beam through the homogenizer.
The extent of the fall in the intensity distribution increases with the number of inverted sub-regions or sub-beams, and thus can be controlled by the number of inverted sub-beams.